Particle detection is a useful tool in many fields of art. In some instances, one or more particles are detected because of an effect they cause when passing through a medium, such as a gas medium. In some applications, a gas is ionized by interactions with a passing or detected particle. The ionization of a detector gas can be amplified with electromagnetic fields and a detector current or signal can be sensed corresponding to the detection of the passing interacting particle. While the present embodiments and examples can be used for detecting muon particles, those skilled in the art will appreciate that the present designs and principles can also be applied to particle (e.g., neutrons, X-Rays, etc.) detection beyond just muon detection. For example, by adding a “neutron conversion” layer of 6Li or 10B just before the detector detailed here.
A THGEM (Thick Gas Electron Multiplier) detector is a robust, simple to manufacture, high-gain gaseous-electron-multiplier detector. Its operation is based on gas multiplication within small, sub-millimeter to one millimeter, diameter holes, in a double-face Cu-clad printed circuit board (PCB). The Cu on each side of the PCB serves as an electrode. The holes are usually made into the PCB, and we refer to this component as a drilled board (DB). An electric potential is applied between the electrodes and creates a strong dipole electric field within the holes, projecting into the adjacent volumes. This shape of the field is responsible for an efficient focusing of ionization electrons into the holes and their multiplication by a gas avalanche process.
In operation, charged particles pass through the gas and ionize it as Minimum Ionizing Particles (MIP). The electrons that result from ionizations in the gas gap above the THGEM drift towards the THGEM holes. The strong dipole electric field established in the holes by the potential difference between the two THGEM faces pulls the electrons into the holes, where they are multiplied in the strong electric field (e.g., 1-5 MV/m). An extraction field in the gap below the THGEM is responsible for the charge collection onto a readout anode/pad.
THGEMs can operate at various gas mixtures such as Ar-, Ne-, or He-based gas mixtures. Examples of mixtures are ArCO2 (90%, 10%) or NeCF4 (95%, 5%). However the final selection of the gas mixture depends on the characteristics of the particles, and the application requirements. In case of cosmic rays muons detection, the actual detector gain and its dynamic range are important. The energy deposited by cosmic ray muons in the gas, which is amplified in the DB, is distributed in a Landau distribution. This means that most of the pulses have a low amplitude and require a high gain to achieve high detection efficiency. However there are sporadic large pulses due the high-energy tail of the distribution that can trigger discharge at high gain of the detector. High dynamic range can be achieved by using Ne or He based mixtures, as they offer higher dynamic range than Ar based mixtures. Ar based mixtures can be used when low gas volumes are needed, as they generates ×3 more electrons primaries than Ne based mixtures. However, it is more difficult to avoid discharges (in a THGEM configuration) with using Ar based mixtures.
Imaging requires that the 2D location of the multiplied charge is read out. This is done by reading each coordinate separately, which requires about three times more charge than readout using a pad, and hence better amplification. When the charge is collected directly by the readouts in THGEM, the typical solution is to place thick conductive strips on a PCB, and on them, place thinner strips, each comprising of a conductor and below it an insulator, so that both layers of conductors face the gas. Standard industrial PCB manufacturing techniques may not be able to produce such complex structures. Another solution uses the Resistive Well configuration described below and a two-sided readout PCB, with one side reading the x-coordinates and the other reading the y-coordinate. In this solution, neither layer of readout conductors faces the gas, nor the signals are purely inductive.
When the gas volume below the circuit board and above the readout is omitted, together with the cathode that faced this gas volume, the resulting configuration is known as a well, or a “THWELL.” Though the well configuration gives up on the (small) amplification that happens just below the holes in a THGEM, it has the potential to provide stronger signals as the entire avalanche reaches the readout board below. It also simplifies the mechanical structure of the detector and results in a thinner detector. However, since the discharge is cleared through the readouts (and thus, the readout electronics), detectors built in this configuration are likely to be damaged by sparks and, thus, are unreliable.
A variation of THGEM known as Resistive Well includes a resistive layer and an insulating layer below the resistive layer, so the readout is purely inductive. The relevant resistivity for Resistive Well is surface resistivity as the charges collected from the gas are removed to the sides. The Resistive Well configuration can reduce sparking, though the induced charges are somewhat smaller and less focused than the avalanche charges produced in the gas.
Another variation of THGEM known as Resistive Plate Well introduces a resistive plate between the holes and the readout. The relevant resistivity is volume resistivity as the charges collected from the gas are removed through the readout. The Resistive Pate Well configuration can reduce sparking. To ensure good electrical contact between the plate and the readout, the plates are coated with conductive paint and glued to the readouts using conductive epoxy. Using conductive glue is incompatible with detailed readout structures, as needed for 2D readout.
Limitations of a THGEM detector include limited gain and stability due to discharges; centimeter scale boards (e.g., 5 cm) which complicate the mechanical design when trying to develop a large meter scale detector; and large numbers of holes (e.g., pitches between holes of less than 1 mm; hole sizes of about 0.4 mm) which makes the THGEM less cost efficient for larger scale detectors. Manufacturing flaws can occur at the holes due to sharp edges in the copper and the PCB caused by the drilling of the holes, despite attempts to blunt this by etching and other methods to create rims. Hence the yield decreases with the increase in the number of holes, and the price of a working THGEM increases greater than the increase in the number of holes per THGEM. Even if sparking is eliminated, in a Resistive Well or Resistive Plate Well configuration, manufacturing flaws, such as sharp edges, may ionize gas atoms leading to gas avalanches and streamer discharges, which can degrade detection of the signal from the detected particles.
For a large scale muon detector of several square meters, these dimensions and characteristics are not optimized. The small actual size of each THGEM electrode complicates the mechanical design of a large scale detector, influenced the gas distribution and thus distorted the consistency of the measurements. The small holes and the tiny rim affects the stability and gain by generating discharges which limit the electric potential applied between the THGEM electrodes. The relatively thin THGEM board increases the probability of discharges as well as affecting the robustness of the layer—especially for large scale detectors. The relatively small pitch between the holes makes the THGEM less cost efficient for larger scale detectors.
The pitch between holes is a key element in the cost of the THGEM which heavily relies on the number of holes. For example, large a THGEM electrode of one square meter with 1 mm pitch between the holes has a million holes, which makes the THGEM less cost efficient for larger-scale detectors.
It would be desirable to have a detector that overcomes the foregoing limitations.